thumb|Photographic recording of Kα and Kβ X-ray emission lines for a range of elements

Moseley's law is an empirical law concerning the characteristic X-rays emitted by atoms. The law was discovered and published by the English physicist Henry Moseley in 1913–1914. In brief, the law states that the square root of the frequency of the emitted X-ray is approximately proportional to the atomic number: <math display="block">\sqrt \nu \varpropto Z .</math>

History

thumb|upright|[[Henry Moseley, holding an X-ray tube]]

The historic periodic table was roughly ordered by increasing atomic weight, but in a few famous cases the physical properties of two elements suggested that the heavier ought to precede the lighter. An example is cobalt having the atomic weight of 58.9 and nickel having the atomic weight of 58.7.

Henry Moseley and other physicists used X-ray diffraction to study the elements, and the results of their experiments led to organizing the periodic table by proton count.

Apparatus

Since the spectral emissions for the lighter elements would be in the soft X-ray range (absorbed by air), the spectrometry apparatus had to be enclosed inside a vacuum. Details of the experimental setup are documented in the journal articles "The High-Frequency Spectra of the Elements" Part I and Part II.

Results

Moseley found that the <math>K\alpha</math> lines (in Siegbahn notation) were indeed related to the atomic number, Z. An elaborate discussion criticizing Moseley's interpretation of screening can be found in a paper by Whitaker which is repeated in most modern texts.

A list of experimentally found and theoretically calculated X-ray transition energies is available at NIST. Nowadays, theoretical energies are computed with much greater accuracy than Moseley's law allows, using modern computational models such as the Dirac–Fock method (the Hartree–Fock method with the relativistic effects accounted for).

See also

  • Moseley's periodic law, concerning the modern periodic table.
  • Auger electron spectroscopy, a similar phenomenon with increased X-ray yield from species of higher atomic number.
  • Discovery of the neutron Mosley's law was an important step in the development of the understanding of the atom.

References

  • Oxford Physics Teaching - History Archive, "Exhibit 12 - Moseley's graph " (Reproduction of the original Moseley diagram showing the square root frequency dependence)